Fan-shaped layers of dust, for example, accumulate on top of the region's polar ice, spreading in the direction of the prevailing winds.

Scientists have long suspected that spring sunshine warms the carbon dioxide ice, causing it to transform directly into gas and spew dust out like a dirty geyser. (See what a Martian ice "geyser" might look like.)

Now new high-resolution images taken from NASA's orbiting HiRise camera are revealing the delicate internal structure of these gas jets.

The pictures also provide insights into wind patterns and daily cycles on the red planet, scientists say.

Spiders

"Like the Earth, Mars experiences seasons. Unlike the Earth, the polar cap is made out of carbon dioxide, or dry ice," Candice Hansen, of NASA's Jet Propulsion Laboratory, said yesterday at a meeting of the American Geophysical Union in San Francisco, California.

"The carbon dioxide ice is translucent," she added. "That allows sunlight to penetrate and warm the surface below."

The warm dirt transfers heat to the bottom layer of ice, which turns to gas. As pressure builds, the gas scours channels beneath the ice, picking up dust.

When the moving gas finds a hole or breaks through a weak spot, the dust spews into the air and then rains back to the surface downwind.

The new HiRise images clearly show the network of pipelike channels feeding each jet.